1. Field of the Invention
The present invention relates to discriminators for frequency shift keyed signals.
2. Description of the Prior Art
In 2-frequency fsk modulation, an on-off modulating signal is used to switch between a higher (f.sub.H) and a lower (f.sub.L) frequency, usually in a continuous phase manner. The general structure of a discriminator (FIG. 1A) is well known and comprises bandpass filters 1, 2 tuned to the two frequencies f.sub.H, f.sub.L, respectively. The modulus of the filter outputs is taken by rectifiers or (as shown) squaring circuits 3, 4, whose outputs are then subtracted at 5 to provide the demodulated output which may be further filtered at 6 to reduce high frequency ripple. The filtered signal is then converted to a square wave by a thresholding device 7.
It has also been proposed (see U.S. Pat. No. 4,437,068) to employ finite impulse response transversal filters, as illustrated in FIG. 1B. That the input signal is in the form of signal samples is illustrated by a sampler 10.
In the general case, because of the phase-sensitive nature of the FIR filter, quadrature pairs are used with vector summation of the outputs are used. Thus in FIG. 2 the higher frequency is handled by filters 12, 13 having tap coefficients which are sine and cosine functions of the tap number. The filter outputs are squared (14, 15) and summed (16).
Filters, squarers and an adder (17-21) are provided in like manner for the low frequency channel, the coefficients again being sine and cosine function of tap number but with a period corresponding to the lower frequency.
As the filter outputs have already been squared, the outputs are supplied directly to the subtractor 5.